Layer-by-Layer Self-assembly of Co3O4 Nanorod-Decorated MoS2

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Layer-by-Layer Self-assembly of Co3O4 Nanorod-Decorated MoS2 Nanosheet-Based Nanocomposite toward High-Performance Ammonia Detection Dongzhi Zhang,*,† Chuanxing Jiang,† Peng Li,*,‡ and Yan’e Sun† †

College of Information and Control Engineering, China University of Petroleum (East China), Qingdao 266580, P.R. China State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instruments, Tsinghua University, Beijing 100084, P.R. China

‡

ABSTRACT: This article is the first demonstration of a molybdenum disulfide (MoS2)/tricobalt tetraoxide (Co3O4) nanocomposite film sensor toward NH3 detection. The MoS2/Co3O4 film sensor was fabricated on a substrate with interdigital electrodes via layer-by-layer self-assembly route. The surface morphology, nanostructure, and elemental composition of the MoS2/Co3O4 samples were examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy-dispersive spectrometry, and X-ray photoelectron spectroscopy. The characterization results confirmed its successful preparation and rationality. The NH3 sensing properties of the sensor for ultra-low-concentration detection were investigated at room temperature. The experimental results revealed that high sensitivity, good repeatability, stability, and selectivity and fast response/recovery characteristics were achieved by the sensor toward NH3. Moreover, the MoS2/Co3O4 nanocomposite film sensor exhibited significant enhancement in ammonia-sensing properties in comparison with the MoS2 and Co3O4 counterparts. The underlying sensing mechanisms of the MoS2/Co3O4 nanocomposite toward ammonia were ascribed to the layered nanostructure, synergistic effect, and heterojunction created at the interface of n-type MoS2 and p-type Co3O4. The synthesized MoS2/Co3O4 nanocomposite proved to be an excellent candidate for constructing high-performance ammonia sensor for various applications. KEYWORDS: molybdenum disulfide, layer-by-layer self-assembly, nanocomposite film, ammonia sensor, sensing mechanism, heterojunction

1. INTRODUCTION Gas sensors have always been playing an important role in various fields,1 such as industrial production,2−4 automotive industry,5,6 medical application,7,8 indoor air quality supervision,9,10 and environmental monitoring.11,12 The existing gas sensors comprising semiconductor gas sensors,13,14 electrochemical gas sensors,15 catalytic combustion-type gas sensors,16 and optical gas sensors17 are being widely used. Among these, the semiconductor sensor has attracted considerable attention owing to its low fabrication cost, miniature sizes, and ease of integration. Specially, Co3O4 exhibits good gas-sensing characteristics and is one of the most promising p-type semiconductors.18−22 Despite great developments in the Co3O4-based gas sensor, there still exist some shortcomings in terms of poor selectivity, long response and recovery time, and high operating temperature. Recently, considerable effort has been made to improve the gas-sensing performance of Co3O4. Recent developments reported that the enhancement of Co3O4 with some other semiconductors to form heterojunction is an effective method to improve its gas-sensing properties. Nowadays, as an emerging two-dimensional (2D) nanomaterial, molybdenum disulfide (MoS2) has attracted considerable attention in developing gas sensors mainly because of the © 2017 American Chemical Society

distinguished electrical properties and atomically thin-layered nanostructure.23−27 As a kind of n-type semiconductor, intrinsic MoS2 has a similarity to the graphene layer structure, with a natural direct band gap of 1.2−1.9 eV, and large surface-tovolume ratio.28−30 These unique properties could complement graphene and make it a promising candidate for constructing gas sensors. Its rationality has been demonstrated by some researchers.31−36 For instance, Zhao et al. synthesized TiO2 nanotubes modified with MoS2 nanoflakes (TiO2−MoS2) as a composite gas sensor, which showed higher specific surface area than TiO2 nanotubes and exhibited excellent sensing performance toward ethanol vapors.37 Zhang et al. reported an ultrasensitive humidity sensor based on SnO2-modified MoS2 nanocomposite via a facile hydrothermal route, exhibiting a significant enhancement toward humidity sensing in comparison to the pure MoS2, SnO2, and graphene counterparts.38 Yan et al. fabricated ZnO-coated MoS2 composite for ethanol gas detection, and the results showed superior gas-sensing performance.39 It can be expected that the metal oxide Received: December 6, 2016 Accepted: January 31, 2017 Published: January 31, 2017 6462

DOI: 10.1021/acsami.6b15669 ACS Appl. Mater. Interfaces 2017, 9, 6462−6471

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Figure 1. (a) Layer-by-layer fabrication process of MoS2/Co3O4 nanocomposite film; (b) drop-casting fabrication process of MoS2/Co3O4 nanocomposite film.

2. EXPERIMENTAL SECTION

semiconductor modified on the MoS2 nanosheets could render its features as novel gas-sensing nanomaterials. To the best of our knowledge, there is no report that has been published on the MoS2/Co3O4 heterojunction toward gas sensing as a new type of electronic device. In this paper, a facile and cost-effective method of fabricating a MoS2/Co3O4 nanocomposite film sensor on a substrate with microelectrodes has been demonstrated. The nanostructural, surface morphology, and elementary composition of the MoS2/ Co3O4 nanocomposite were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive spectrometry (EDS), and X-ray photoelectron spectroscopy (XPS) measurements. The sensing performances of the MoS 2 /Co 3 O 4 nanocomposite film sensor were investigated at room temperature by exposure to a low concentration of ammonia gas over a range of 0.1−5 ppm. The effect of the number of self-assembled layers on the sensing properties was also explored. Moreover, the MoS2/Co3O4 nanocomposite film sensor exhibited significant enhancement in ammonia-sensing properties in comparison with the MoS2 and Co3O4 counterparts. Experimental results showed that the self-assembled MoS2/Co3O4 film sensor had distinct advantages of high sensitivity, good repeatability, stability, and selectivity and fast response/ recovery characteristics.

2.1. Materials. Reagents used in the experiment including sodium molybdate dehydrate, thioacetamide, oxalic acid, cobalt nitrate hexahydrate, sodium phosphate dodecahydrate and hydrazine hydrate were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification. Polycation and polyanion used for layer-by-layer self-assembly were 1.5 wt % poly(diallyldimethylammonium chloride) [PDDA, MW of 200−350 K, Sigma-Aldrich, St. Louis] and 0.3 wt % poly(sodium 4styrenesulfonate) [PSS, MW of 70 K, Sigma-Aldrich] with 0.5 M NaCl in both for ionic strength. A facile hydrothermal route was used to synthesize MoS2 and Co3O4.40,41 Sodium molybdate dihydrate (1.0 g) and thioacetamide (1.2 g) were dissolved into 80 mL of deionized (DI) water and stirred for 0.5 h. Subsequently, 0.6 g of oxalic acid was added into the above mixed solution and stirred for another 0.5 h. The resulting mixture was hydrothermally treated in a Teflon-lined autoclave at 200 °C for 24 h. The high-purity MoS2 crystalline was obtained by washing with DI water several times, and was then mixed with PSS solution (0.3 wt %). The mixed MoS2−PSS solution was slightly stirred and ultrasonicated for 10 min. The synthesis of Co3O4 nanoparticle was similar to that of MoS2; 0.5 mmol of cobalt nitrate hexahydrate and 0.0125 mmol of sodium phosphate dodecahydrate were dissolved into 70 mL of DI water and then stirred for 0.5 h. Then, 2 mL of hydrazine hydrate was added to the mixture dropwise and ultrasonic agitation was performed for 0.5 h. Subsequently, the resulting mixture was hydrothermally treated at 180 °C for 12 h. Finally, the obtained sample was further annealed at 500 °C in nitrogen for 5 h to get high-quality Co3O4 crystalline. 6463

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Figure 2. SEM image (a) and particle size distribution (b) of Co3O4; SEM image (c) and particle size distribution (d) of MoS2; SEM image (e) and sectional view (f) of MoS2/Co3O4 sample.

Figure 3. TEM images of (a) MoS2 nanosheets and (b) MoS2/Co3O4 hybrid. HRTEM images of (c) MoS2 nanocrystalline and (d) MoS2/Co3O4 hybrid. reported in our previous work.38 The sensing film on the device was deposited by layer-by-layer (LbL) self-assembly, as shown in Figure 1a.

2.2. Sensor Fabrication. The sensor device was fabricated on a polychlorinated biphenyl substrate with interdigitated electrodes, as 6464

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Figure 4. (a) XRD pattern of MoS2, Co3O4, and MoS2/Co3O4 samples. (b) EDS spectrum of MoS2/Co3O4 samples. XPS spectra of MoS2/Co3O4 sample: (c) Mo spectrum of MoS2/Co3O4 sample, (d) S spectrum of MoS2/Co3O4 sample, (e) Co spectrum of MoS2/Co3O4 sample, and (f) O spectrum of MoS2/Co3O4 sample. Two bilayers of PDDA/PSS as precursor layers were first selfassembled on the substrate surface to enhance charge and adsorption capacity, followed by alternative turns of the device immersion into Co3O4 and MoS2−PSS solutions for required cycles. The deposition time used was 10 min for polyelectrolytes and 15 min for Co3O4 and MoS2−PSS. Moreover, an intermediate rinsing with DI water and drying under a stream of nitrogen was required after each monolayer assembly to reinforce the interconnection between layers. Multilayer self-assembled film with the desired number of layers could be built by repeating the immersion cycles. PDDA and Co3O4 provide positive charges, whereas PSS and MoS2−PSS provide negative charges. According to this method, MoS2/Co3O4 films with different layers (one, three, five, and seven layers) were prepared under the electrostatic force interaction. To highlight the sensing properties of LbL self-assembled MoS2/Co3O4 film sensor, another MoS2/Co3O4 nanocomposite sensor was fabricated as a contrast by drop-casting their mixture solution onto the electrodes, as shown in Figure 1b. 2.3. Instrument and Analysis. The ammonia-sensing experimental measurement was performed at room temperature and relative humidity of 40% RH. The sensing properties of the self-assembled film sensors upon exposure to various concentrations of ammonia gas were investigated. The desired gas concentration was obtained through injecting a certain volume of ammonia gas into a sealed container with a syringe, and the sensor was placed inside the container.42,43 The variation of the ammonia concentration is converted into a corresponding resistance change of the sensors, which was measured

using a data logger (Agilent 34970A). Normalized response S was used to evaluate the sensor performance, which was defined by S = |Rx − R0|/R0 × 100%, where R0 and Rx were the measured resistance of the sensor in air and NH3 gas, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization Results. The surface morphology of as-prepared Co3O4, MoS2, and MoS2/Co3O4 samples was observed using field emission scanning electron microscopy (SEM; Hitachi S-4800). Figure 2a shows that the hydrothermally synthesized Co3O4 has nanorod shape. The size distribution for the Co3O4 was determined by software of Nano Measurer as shown in Figure 2b, indicating an average diameter of 98.15 nm. Figure 2c illustrates that MoS2 has a well-defined nanoflake-shaped structure, and its size distribution (Figure 2d) confirmed an average diameter of 197.43 nm. Figure 2e illustrates that the Co3O4 nanorods and MoS2 nanosheets contacted each other in the MoS2/Co3O4 nanocomposite. Figure 2f shows the sectional view of MoS2/Co3O4 sample. The transmission electron microscope (TEM; JEOL JEM2100) was used to examine the nanostructure of the asprepared samples. Figure 3a,b shows the TEM images of the MoS2 and MoS2/Co3O4 nanocomposite. Figure 3c,d shows the typical high-resolution TEM (HRTEM) images of the samples. 6465

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ACS Applied Materials & Interfaces Figure 3c shows the lattice fringe spacing of 0.620 and 0.273 nm for MoS2 crystalline, which is attributed to the (002) and (100) planes of the MoS2.39,44 Figure 3d shows Co3O4 nanorods on MoS2 nanosheets with labeled lattice fringes of 0.244 and 0.466 nm, attributed to the (311) and (111) planes of the Co3O4 nanorods.45 The XRD measurement of MoS2, Co3O4, and MoS2/Co3O4 samples was performed with an X-ray diffractometer (Rigaku D/Max 2500PC). All the synthesized samples were inspected with diffraction peaks in a range of 10°−70°, and the plots of the XRD spectrum are shown in Figure 4a. The diffraction peaks of MoS2 nanosheets are observed at 2θ of 14.4°, 33.1°, 38.5°, and 59.3°, which correspond to the (002), (100), (103), and (110) planes of the MoS2 nanocrystal. The XRD spectrum of Co3O4 nanocrystal shows the feature peaks observed at different planes for the typical cubic crystal structure (JCPDS 65-3103).46 The XRD pattern of the MoS2/Co3O4 nanocomposite substantially contains the main characteristic peaks of the MoS2 and Co3O4, confirming the presence of MoS2 and Co3O4 in the MoS2/Co3O4 nanocomposite. To explore the elementary composition, the energy-dispersive spectrometer (EDS, Hitachi S-4800) was used to further characterize the MoS2/Co3O4 sample. The EDS spectrum is shown in Figure 4b. Only the elements Mo, S, Co, and O are detected in the MoS2/Co3O4 hybrid; no other impurity element is observed. The surface compositions and chemical states of the MoS2/ Co3O4 nanocomposite are measured by a Thermo Scientific KAlpha XPS spectrometer. Figure 4c shows the Mo 3d spectrum of the nanocomposite. Two peaks at 229.09 and 231.28 eV are ascribed to the doublet Mo 3d5/2 and Mo 3d3/2 of Mo4+ from MoS2.47 Other peaks shown at 226.05 and 235.38 eV attribute to S2− 2s and Mo 3d3/2 of Mo6+, which suggests that the sample possibly contains an amount of MoO3.37 The spectra of S 2p shown in Figure 4d exhibit two main double peaks at 161.88 and 163.28 eV, which correspond to S 2p3/2 and S 2p1/2 of S2−, respectively.29,48 Figure 4e shows the atom of Co in the sample has two valence states, tetrahedral Co2+ and octahedral Co3+ from the Co3O4. The major peaks of Co3+ at 781.18 and 796.78 eV contribute to the Co3+ 2p3/2 and Co3+ 2p1/2 respectively. And, Co2+ also has two major peaks at 786.98 and 803.08 eV, which are attributed to the Co2+ 2p3/2 and Co2+ 2p1/2, respectively.49 Figure 4f shows the XPS of O 1s, indicating two major peaks at 530.64 and 529.50 eV in correspondence with the oxygen species of Co3O4 and MoO3.18,37 3.2. NH3 Gas-Sensing Properties. Figure 5 shows the NH3 gas-sensing response of self-assembled MoS2/Co3O4 film sensors with different layers. To investigate the effect of selfassembled layers, the four sensors with one, three, five, and seven layers were labeled as S1, S3, S5, and S7, respectively. The measurement was conducted by exposing the four sensors to ammonia gas over a range of 0.1−5 ppm. The timedependent gas-sensing response−recovery curves for the four sensors switched between different concentrations of ammonia and air are shown in Figure 5a. The responses of these sensors are remarkable when they are switched from air to NH3 environment, and increase with the increasing concentration of ammonia gas. Each exposure/recovery cycle is 200 s. Rapid response and recovery characteristics can be observed for these sensors. Figure 5b plots the sensing responses of these sensors as a function of gas concentration. Among them, the sensor with five layers (S5) yielded the highest response, and the corresponding response values are determined to be 8.74, 12.28, 16.86, 26.14, 44.35, and 63.78% with ammonia gas

Figure 5. NH3 gas-sensing properties of LbL self-assembled MoS2/ Co3O4 nanocomposite sensors with different layers: (a) Timedependent gas-sensing response toward different concentrations of NH3; (b) the sensor response as a function of gas concentration.

concentrations of 0.1, 0.25, 0.5, 1, 2, and 5 ppm. This is because of the low conductivity of the sensors with fewer layers, and difficulty in gas diffusion for the sensors with more layers.50 This investigation reveals the influence of the number of selfassembled layers on the sensing properties of the MoS2/Co3O4 nanocomposite. Therefore, the MoS2/Co3O4 sensor with five layers was employed in the following experiments. Figure 6 shows the resistance measurement of the MoS2/ Co3O4 film sensor upon exposure to cumulative ammonia

Figure 6. Resistance measurement of the LbL self-assembled MoS2/ Co3O4 nanocomposite sensor toward a step increase of ammonia concentration and recovery in air.

concentrations with 1 ppm as the step increase. The exposure time for each concentration is ∼100 s, and the measuring range is from 1 to 6 ppm. The result indicates that the resistance monotonically increased as the concentration of ammonia gas rose, and also shows a good recovery characteristic in air to the initial resistance values. Figure 7 shows the repeatability of the MoS2/Co3O4 film sensor upon exposure to 0.25, 0.5, and 1 ppm of ammonia gas. The measurement was performed under the same conditions for five exposure/recovery cycles. A good consistency and 6466

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Figure 7. Repeatability of the MoS2/Co3O4 film sensor upon exposure to 0.25, 0.5, and 1 ppm of ammonia gas.

reproducibility can be observed from the measurement results. Figure 8 shows the selectivity of the MoS2/Co3O4 film sensor

Figure 9. (a) Normalized response and (b) function fitting of the MoS2/Co3O4, MoS2, and Co3O4 film sensors toward various ammonia concentrations.

Figure 8. Selectivity of the MoS2/Co3O4 film sensor upon exposure to ammonia (NH3), ethanol (CH3CH2OH), benzene (C6H6), formaldehyde (HCHO), and acetylene (C2H2) with concentrations of 5 and 2 ppm, respectively.

upon exposure to ammonia (NH3), ethanol (CH3CH2OH), benzene (C6H6), formaldehyde (HCHO), and acetylene (C2H2) at room temperature. Concentrations of 5 and 2 ppm for the above gas species were measured under the same conditions. The MoS2/Co3O4 film sensor exhibited much higher sensitivity to NH3 gas than those of other tested gases, and an excellent selectivity was observed. To highlight the merit of the LbL self-assembled MoS2/ Co3O4 film sensor, its sensing properties toward ammonia were compared with that of MoS2 and Co3O4 film sensors. The three sensors were tested upon exposure of ammonia over a range of 0.1−5 ppm. Figure 9a shows the normalized response of the MoS2/Co3O4, MoS2, and Co3O4 film sensors. It is clear that the MoS2/Co3O4 film sensor exhibits the highest response among the three sensors. Figure 9b plots the normalized response of the MoS2/Co3O4, MoS2, and Co3O4 film sensor as a function of ammonia gas concentration. The fitting equations for the sensor response Y and gas concentration X are represented as Y = 10.236X + 13.544, Y = 3.453X + 5.064, and Y = 1.213X + 2.332 for the MoS2/Co3O4, MoS2, and Co3O4 film sensor, respectively, and the regression coefficient, R2, is 0.8853, 0.9023, and 0.9114, respectively. Fabrication methods may affect the gas-sensing properties of the sensors. The sensing performances of LbL self-assembled and drop-casted MoS2/Co3O4 film sensor toward 5 ppm ammonia gas were measured and compared, as shown in Figure 10. The response of the LbL self-assembled sensor is twofold compared to that of the drop-casted sensor. The response time

Figure 10. Sensing performances of LbL self-assembled and dropcasted MoS2/Co3O4 sensor toward 5 ppm ammonia gas.

of LbL self-assembled sensor is about 98 s and the recovery time is about 100 s, whereas the response/recovery time of the drop-casted sensor is around 105 s/136 s. The result highlights the LbL self-assembled sensor features better sensing properties toward ammonia detection such as higher response and swifter response/recovery characteristics, compared to the drop-casted sensor. This is because of the layered nanostructure deposited by LbL self-assembly method and special interaction at the heterojunction interfaces. We presented the basis resistance measurements of different layer assembled structure, the MoS2 film and Co3O4, and the dropped composite in Table 1. The sensors with different layers have different resistance. The response of different sensors was calculated by the relative resistance change ΔR/R0 × 100%. We listed the response of the sensors toward 5 ppm ammonia gas for different sensors in the Table 1. To evaluate the gas diffusion in sensor material, we fit the sensor response curve 6467

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ACS Applied Materials & Interfaces Table 1. Performance Survey of the Sensors Presented in This Work sensors

fabrication method

sesistance in air

resistance in NH3 (5 ppm)

response

S1 S3 S5 S7 MoS2 sensor Co3O4 sensor MoS2/Co3O4

self-assembly self-assembly self-assembly self-assembly drop casting drop casting drop casting

54.41 MΩ 24.71 MΩ 11.61 MΩ 2.31 MΩ 0.27 MΩ 114.88 MΩ 18.3 MΩ

56.15 MΩ 28.69 MΩ 18.86 MΩ 2.99 MΩ 0.22 MΩ 124.3 MΩ 22.8 MΩ

3.2% 16.1% 62.4% 29.4% 18.5% 8.2% 24.6%

τ 39.53 40.11 41.90 42.52 44.39 45.64 42.73

s s s s s s s

Table 2. Performance of the Presented Sensor in This Work Compared with Previous Work sensing material

fabrication method

operating temperature

rGO/Co3O4 MoS2/Au MoS2/Si rGO/ZnO rGO/TiO2 rGO/Cu2O SnO2/graphene rGO/Ag MoS2/Co3O4

electrospinning drop coating dc magnetron sputtering spraying drop coating drop coating drop coating spray coating LbL self-assembly

RT 60 °C RT RT RT RT RT RT RT

detection limit 5 25 200 10 10 100 10 15 0.1

ppm ppm ppm ppm ppm ppm ppm ppm ppm

response

ref

3% 44.4% 19.1% 1.2% 1.6% 74.7% 5.8% 3.1% 10.3%

18 26 52 53 54 55 56 57 this work

Figure 11. Schematic of (a) sensing mechanism and (b) energy band structure diagram and resistance variation for the n-type MoS2/p-type Co3O4 hybrid in air and ammonia. (Ec, conductor band; Ev, valence band; Ef, Fermi level).

with an equation by R = Rf + (R0 − Rf)e−t/τ.51 Here, R is instantaneous resistance of the sensor, R0 is the initial resistance of the sensor, Rf is the final resistance of the sensor, t is the time, and τ is the time constant related to diffusion rate. The time constants for the sensors with different structures were determined based on the sensor response data. The time constant corresponding to the sensor upon exposure to 5 ppm ammonia was listed in Table 1. We find that the time constant is increased with the increasing of the self-assembled layers, indicating the thinner film layer facilitates the gas diffusion.50

Table 2 presents the ammonia gas-sensing properties of the MoS2/Co3O4 film sensor compared with those of the published works.18,26,52−57 The comparison highlights this work in which much higher performance in terms of low detection limit and high response compared to that of the much reported MoS2, reduced graphene oxide (rGO), and metal oxide-based counterparts. The MoS2/Co3O4 nanocomposite has undoubtedly proved to be an excellent candidate for constructing highperformance ammonia sensor for various applications. 3.3. Ammonia-Sensing Mechanism. The above-mentioned experimental results confirmed that the MoS2/Co3O4 6468

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ACS Applied Materials & Interfaces film had excellent sensing properties toward ammonia gas at room temperature, such as high sensitivity and selectivity, good repeatability and stability, and fast response/recovery characteristics. Figure 11a shows the proposed mechanism for the sensing behavior of the MoS2/Co3O4 film sensors toward ammonia gas. The layered nanostructure not only effectively prevents the agglomeration but also creates many more contact sites. The p-type Co3O4 nanorods (NRs) act as active catalytic sites toward ammonia gas.58 When the sensor is exposed to air, the oxygen molecules absorb onto the surface of Co3O4 NRs and form O2− (ads) through capturing free electrons from the conduction band of Co3O4. A hole accumulation layer (HAL) is created in the surface of the Co3O4 NRs because of the consumption of electrons in the Co3O4 NRs.23−27 Oxygen molecules act as electron acceptors in this process, which lead to the increased concentration of holes and decreased sensor resistance. When the sensor is exposed to ammonia gas, NH3 molecules react with the oxygen ions and release the trapped electrons back to the conduction band, resulting in the decrease of hole accumulation layer width and the increase of the measured resistance of the sensor. The sensing reaction can be represented as follows:57,59

O2(gas) → O2(ads) O2(gas) + e →

O2(ads)−

4NH3 + 5O2(ads)− → 4NO + 6H 2O + 5e−

fabricated using LbL self-assembly technique. The surface microscopy, nanostructure, and elementary composition of the as-prepared MoS2/Co3O4 nanocomposite were characterized by SEM, TEM, XRD, EDS, and XPS. The ammonia-sensing properties of the MoS2/Co3O4 film were investigated at room temperature. The sensor exhibited high sensitivity, good repeatability, stability, selectivity, and fast response/recovery characteristics against low-concentration detection of ammonia, which was superior to the Co3O4 and MoS2 film sensors. Finally, the underlying ammonia-sensing mechanism of the sensor was discussed based on the experimental results. The experimental results highlight the fact that the LbL selfassembled MoS2/Co3O4 film is a promising candidate for building ammonia gas sensors.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86-532-86981813, ext. 426. Fax: +86-532-86981335 (D.Z.). *E-mail: [email protected] (P.L.). ORCID

Dongzhi Zhang: 0000-0001-9238-4176 Notes

(1)

The authors declare no competing financial interest.

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(2)

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51407200, 51405257), the Science and Technology Plan Project of Shandong Province (Grant No. 2014GSF117035), the Fundamental Research Funds for the Central Universities of China (Grant No. 15CX05041A), and the Science and Technology Development Plan Project of Qingdao (Grant No. 16-6-2-53 nsh).

(3)

MoS2 is an n-type semiconductor with natural band gap of 1.2−1.9eV, and Co3O4 behaves as a p-type semiconductor with band gap of 2.07 eV. As we all know, p- and n-type semiconductors are dominated by holes and electrons, respectively. When they contact each other, the interdiffusion of both dominant carriers at the interface leads to the formation of a depletion layer, namely, p−n heterojunction. Figure 11b shows the schematic energy diagram of the p−n heterojunction created at the interface of p-type Co3O4 and n-type MoS2. The holes in Co3O4 and the electrons in MoS2 forms a self-built electric field at the heterogeneous region, and a depletion layer can be established when they realize an equalization of the Fermi level. The modulation of potential barrier formed at the MoS2/Co3O4 interface occurs owing to the adsorption and desorption of ammonia.37−39 Figure 11b shows the variation of the depletion layer width for the MoS2/Co3O4 nanocomposite film upon exposure to air and ammonia gas. In air, the density of holes on the surface of Co3O4 increases, whereas the electrons on the surface of MoS2 decrease because of the ionization of absorbed oxygen species.60 The transferring of holes from MoS2 to Co3O4 leads to the contraction of the depletion layer, resulting in a decrease of resistance for the sensor. When the sensor was exposed to ammonia, the interaction between adsorbed O2− and ammonia molecules (eq 2) releases free electrons and neutralizes the holes in the Co3O4, which contributes to the expansion of the depletion layer and results in an increase in sensor resistance.61 This is the main reason for the MoS2/Co3O4 composite sensor exhibiting superior sensing performances in terms of high sensitivity, and fast response and recovery characteristics, which is much better than that of MoS2 and Co3O4 sensors, as shown in Figure 9.

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REFERENCES

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